1. Field of the Invention
The present invention relates to blocking gels of the type used in well bore operations and particularly to a method for producing a gradual reduction in the viscosity of a blocking gel through the use of enzymes either incorporated internally within the blocking gel or placed externally on the blocking gel.
2. Description of the Prior Art
Occasionally, production from well bore operations must cease temporarily to perform auxiliary procedures such as repairs at different depths of a subterranean formation. The repairs are called workover operations. Workover operations frequently use heavy brines and other fluids to maintain pressure control within the reservoir. The fluids can leak-off into the production zone, causing damage which interferes with the efficient operation of the well.
Isolating the production zone, however, protects it from damage. Specific blocking materials such as solid blocking agents or temporary blocking gels isolate the production formation. The solid blocking agents, for example nylon or rubber balls, are injected into the fluid stream and seal the production formation by physically stopping up perforations in the formation. When the injection ends, the material is no longer held against the perforations and falls to the bottom of the well.
The use of temporary blocking gels successfully protects the production zone. Blocking gels form by gellation of suitable polymers, such as appropriate polysaccharides. These gels produce a relatively impermeable barrier across the production formation. The barrier cordons off the production zone from the area undergoing the workover operations. The areas must remain separated until production is ready to resume.
Production resumes after removal of the blocking gel. The recovery of the blocking gel is accomplished by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion is referred to as "breaking" and is often accomplished by incorporating chemical agents, referred to as breakers, into the initial gel.
In addition to the importance of providing a breaking mechanism for the gelled fluid which facilitates recovery of the fluid and resumes production, the timing of the break is of great importance. Gels which break prematurely can damage the production zone through the leak-off of contaminating materials into the production formation.
On the other hand, gelled fluids which break too slowly can cause slow recovery of the blocking gel fluid from the production formation. Slow recovery delays the resumption of the production of formation fluids. Incomplete gel degradation causes a build up of residue, forming a filter cake which interferes with production from the formation.
For purposes of the present application, premature breaking will be understood to mean that the gel viscosity becomes diminished to an undesirable extent prior to the end of the workover operation. Thus, to be satisfactory, the gel viscosity should remain in the range from about 60% to 100% for the length of time required to complete the workover operation. Since some workover operations require extended periods of time before completion, the blocking gels should be capable of remaining appropriately viscous during that time period.
Optimally, the blocking gel will break when the auxiliary operations conclude. For practical purposes, the gel should be completely broken within a specific period of time after completion of the auxiliary operations. This period of time is dependent on the temperature of the formation. A completely broken gel means one that can be flushed from the formation by the flowing formation fluids. In the laboratory setting, gel viscosity is measured using a rotational viscometer such as a Fann 35VG meter or a Brookfield DVII digital viscometer. A completely broken, noncrosslinked gel regains greater than about 95% of the initial permeability of a formation sample using a gel damage permeability test.
By way of comparison, certain gels, such as those based upon guar polymers, undergo a natural break without the intervention of chemical additives. The breaking time for such gelled fluids is excessive, being somewhere in the range from greater than 24 hours to about two weeks at an exposure temperature of about 80.degree. F. Accordingly, to decrease the break time of blocking gels, chemical agents are incorporated into the gel and become a part of the gel itself. Typically these agents are either oxidants or organic materials which degrade the polymeric gel structure.
However, obtaining controlled breaks using various chemical agents, such as oxidants or organic materials, has proved difficult. Common oxidants, for example persulfates, are ineffective at low temperature ranges from ambient temperature to 130.degree. F. In this temperature range the oxidants are stable and do not readily undergo homolytic cleavage to initiate the degradation of the polymer structure. A gel break is typically achieved at lower temperatures only through the addition of high concentrations of oxidizers or the addition of a coreactant to initiate cleavage. High oxidizer concentrations are frequently poorly soluble under the treatment conditions. High oxidizer concentrations interfere with the polymer crosslinking, consequently forming unstable gels that can prematurely break. Common oxidants also break the blocking gel's polysaccharide backbone into nonspecific units, creating a mixture of monosaccharide, disaccharide and polysaccharide fragments as well as miscellaneous fragments. Further, common oxidants are difficult to control. Oxidants react with things other than the polymeric gel. Strong oxidizers react with the metals used for crosslinking the polymer, consequently weakening the gel and reducing long term stability. Oxidants can react with iron found in the formation, producing iron oxides which precipitate and damage the formation. Oxidants can also react nonspecifically with other materials used in the oil industry, for example, tubings, linings and resins.
To increase the efficiency of common oxidizers at lower temperatures, coreactants are often used as catalysts. One group of coreactants are antioxidizers, such as triethanolamine. Using antioxidants presents two additional problems. First, antioxidants such as triethanolamine are expensive to use in the quantities required for well operations. Secondly, internally incorporated antioxidants initiate breaks almost immediately. These are often rapid breaks which lead to the premature reduction of gel viscosity while degrading the gel incompletely, thereby damaging the production zone or allowing injection of the workover fluid.
Rather than using common oxidants, organic materials have been tried. One such material is sucrose. Sucrose gives slow internal breaks. Successful gel breaking requires high concentrations of sucrose, usually in excess of 600 pounds of sucrose per thousand gallons of gel. Sucrose can be difficult to solubilize at these high concentrations. The reduction of gel viscosity is also unsatisfactory. The ge is reduced only to the initial viscosity of the solution prior to gellation, reducing the efficiency of the recovery.
A second organic material tried is polyglycolic acid. Polyglycolic acid produces a slow reduction of viscosity of the gel. But this reduction breaks the gel incompletely, often damaging the formation.
To produce complete breaks with oxidants, sucrose or polyglycolic acid as internal breakers, additional treatment may be required. An extra acid hydrolysis step may be necessary to remove residual polymer residue. Treatment with an acid for example, hydrochloric acid, augments the gel breaking for complete breaks. Acid treatments corrode steel and equipment used in the operation.
Enzyme systems are known to degrade the types of polysaccharides used in blocking gels. Enzyme breaker systems have been designed to break gelled fracturing fluids used in the industry. See, for example, the copending application of Robert Tjon-Joe-Pin entitled "Enzyme Breaker For Galactomanna Based Fracturing Fluid", Application No. 07/842,038, filed Feb. 26, 1992, now U.S. Pat. No. 5,201,370, filed concurrently herewith. Enzymes, for example the cellulases, hemicellulases, amylases, pectinases, and their mixtures are familiar to those in the well service industry when used in fracturing gel breaker systems. These enzymes break the bonds that connect the monosaccharides into a polysaccharide backbone, for instance, the (1,4)-.alpha.-D-galactosiduronic linkages in pectin. These conventional enzymes are nonspecific and cause random breaks. Therefore using these conventional enzymes to break gelled fracturing fluids results in only a partial degradation of the polysaccharide polymer. Instead of fragmenting almost completely into much smaller fragments, these enzymes break the polysaccharide backbone into a mixture of fragments consisting of monosaccharides, disaccharides and polysaccharides. Larger fragments like disaccharides and polysaccharides can plug the formation. Since the breaks are nonspecific, conventional enzyme breakers can degrade other components used in the system.
The same results occur when used in a blocking gel system. The larger fragments increase the residue left in the formation once the gel is removed. Increased residue can damage the formation and decrease the productivity of the well bore operation by plugging the formation. For example, in a cellulose based system conventional breakers can only break the gel down to predominately polysaccharides. Traditional enzyme breaker systems may be able to degrade the cellobiose units further. However, this degradation is too slow and inadequate.
Incomplete breakdown of the gel can damage the production zone. Inadequately broken blocking gels produce formation permeability damage. Increasing the concentration of breakers to prevent incomplete breaking, decreases the amount of insoluble residue. Unfortunately, this may also increase gel instability, precluding long term gels.
The present invention has as its object to provide a break mechanism for a blocking gel which yields high initial viscosity with little change during auxiliary operations but which produces a rapid break in the gel after auxiliary operations are completed to allow immediate recovery of the fluid from the formation.
Another object of the invention is to provide a system for a blocking gel which can break the gel polymers within a wide range of pH at low to moderate temperatures without interfering with the crosslinking chemistry.
Another object of the invention is to provide an enzyme breaker system which breaks the crosslinked polymer backbone primarily into monosaccharide fragments.
Another object of the invention is to provide an enzyme breaker system for a blocking gel which produces a controlled break at low to moderate temperatures and which decreases the amount and size of residue left in the formation after recovery of the fluid from the formation.